Many winter-active temperate rodent species show seasonal, photoperiod-mediated changes in body mass that correlate with changes in food availability, gaining mass when food is abundant. The collared lemming (Dicrostonyx groenlandicus), an arctic species, also undergoes a seasonal transition in body mass, but one that is temporally out of phase with that observed in species from lower latitudes; whereas temperate species increase in mass when exposed to relatively long day lengths, collared lemmings do so under long but decreasing day lengths. This adaptation may be in response tothe timing of peak above-ground biomass in the arctic. Validation of this hypothesis requires a more thorough examination of both collared lemmings and other arctic species.In the collared lemming, photoperiod-mediated changes in body mass are correlated with changes in serum concentrations of prolactin, thyroid hormones, corticosterone, and growth hormone, and pineal concentrations of melatonin. These observations are compared to those obtained with temperate small mammals, and possible cause-and-effect relationships between body mass and hormonal parameters are discussed. © 1995 by the American Society of Zoologists.